This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to CN99211181.1 filed in China on May 24, 1999; the entire content of which is hereby incorporated by reference.
This invention relates to a pair of glasses used for short distance activities for extended periods of time. By wearing it, the eyestrain and acquired myopia can be reduced or prevented.
In ancient times people always engaged in hunting, plowing and weeding under the wide natural environment, and the eyes were primarily used for viewing distance. But along with the enhancement of productivity and the progress of society, changes have been taking place in labour mode and life style. Meanwhile, the outdoor activities have been being gradually replaced by indoor close activities. Because of sustained viewing near object and continuous accommodation and convergence, a series of symptoms of eyestrain occur, such as giddiness, photophobia, diplopia, afterimages, blurred vision, etc. As a result of the combined pulling and oppressing effect of intraocular and extraocular muscles, the eyeball axis gets longer and the radius of cornea curvature becomes shorter, so myopia occurs. In order to reduce eyestrain and prevent myopia, people make a suggestion for sustained near work: taking some rests and far-gazing, keeping-fit massage of eye, proper illuminating for reading and writing, printing in sharp contrast, seating in right posture, keeping proper viewing distance for reading and so on. These measures are not doubt benefit to vision protection, but can far not shake the foundation giving rise to eyestrain and myopia xe2x80x94sustained viewing near object and continuous accommodation and convergence. Some investors presented a kind of glasses having base-in prism lenses in their patents (U.S. Pat. No. 5,076,665 to Mardian, U.S. Pat. No. 5,204,702 to Shapiro, U.S. Pat. No. 5,381,191 to Levy). The purpose of those inventions is for reducing eyestrain. However, those glasses are not very satisfied for long-period wearing due to overweight. Though the glasses mounted on computer monitor can get rid of wearing problem, they restrict the head movement freedom, so as to bring about some new problems such as muscles ache of neck and back, physical fatigue and so on.
An object of the present invention is to provide an optical measure to transform the near object view into far object view, so as to eliminate the primary postnatal environment factor causing eyestrain and myopia.
Another object of the present invention is to provide a pair of light glasses for sustained reading or watching at computer monitor screen to reduce eyestrain.
A further object of the present invention is to provide a pair of glasses allowing eyes in orthophoric state in clear focus without accommodation and convergence when watching at close object.
A still further object of the present invention is to provide a method by means of which one can choose a pair of suitable inventive glasses easily on basis of different diopter of each individual.
The inventive glasses can be implemented as follows: The said convex lens and stepped prism in a unitary configuration are made of thermosetting optical resin, such as allyl diglycol carbonate (CR-39), or thermoplastic optical resin, such as acrylic resin, by injection-mold way. Through lens-hardening, edge-grinding and framing, the glasses having lenses of front convex surface and back stepped prism with its base towards the nose will then be made.
The dioptric power of the lens is determined by the object-viewing distance. While measuring the object-viewing distance d, the interpupillary distance de and the distance dg from eye to lens of glasses by the meter, the dioptric power of the lens Di by the diopter (D) and the dioptric power of the prism Dp by the prism diopter xcex94, the relations for emmetropia with or without presbyopia are:
D1=1/(dxe2x88x92dg)xe2x80x83xe2x80x83(I) 
Dp=100(0.5 de/d)xe2x80x83xe2x80x83(II) 
For example, if the object-viewing distance for reading is 0.3 m and the interpupillary distance is 0.06 m, the dioptric power of the lens and the prism should be +3.5D and 10xcex94 respectively. While operating the computer, the screen-viewing distance is about 0.5 m, so the dioptric power of the lens and prism are equal to +2.0D and 6xcex94 respectively. The above mentioned object-viewing distance indicates the distance between the eye and the gazed object point. In order to simplify the calculation of the dioptric power of the stepped prism, the object-viewing distance is used instead of the direct distance from the eye to the gazed object plane in equation (II). The above said prism diopter xcex94 is an optometric terms, namely, one xcex94 means that the light beam through a prism deflects one centimeter at the place of 1 meter away.
By wearing the selected diopter glasses, the eyes can view the object at or within the predetermined distance without or with a little accommodation and convergence. Therefore, the eyestrain from sustained watching at close object will be greatly reduced. As for ametropia (myopia or hyperopia), the calculating method of diopter power of the stepped prism of the said glasses is as same as that for emmetropia, but the calculating method of dioptric power of the lens is different. Suppose Da is the required diopter of the lens for ametropia, Dr is the diopter of the distance-viewing prescription for said ametropia, and Dl is the lens diopter required for emmetropia, the Da is equal to the algebraic sum of Dl and Dr, the relationship among them can be expressed as:
Da=Dl+Drl xe2x80x83xe2x80x83(III) 
For example, if the diopter of distance-viewing prescription for myopia is xe2x88x923.0DS, the selected lens diopter of said glasses for object-viewing distance of 0.3 m and 0.5 m is +0.5DS and xe2x88x921.0DS respectively. Thus it can be seen that sometimes the lenses of said glasses for myopia are not convex but concave. For another example, if the diopter of distance-viewing prescription for hyperopia is +3.0DS, the diopter of selected lenses of said glasses for object-viewing distance of 0.3 m and 0.5 m is +6.5DS and +5.5DS respectively.
The step numbers of stepped prism in this invention are equal to or more than 3. The more the number of steps, the thinner the base of the stepped prism is and the lighter the stepped prism is. But too more numbers of steps may decrease the clarity due to interference of vertical line and resulting manufacturing difficulties, so that 5-35 steps or the interval distance being 1.5 mm-10 mm are appropriate.
The advantages of this invention are as follows: (1) Said glasses are comparatively light. (2) When wearing said glasses, one can gaze at near object, without accommodation and convergence as gaze at far object. So that the strain of intraocular and extraocular muscles, giddiness, ocular blood circulation obstacle and the eyeball-deforming force caused by near reflex when being engaged in sustained close range work would not occur. (3) The normal function of accommodation and convergence will not be affected because these said glasses are not worn for short-time close work or intermittent close range work but only for sustained close range work. (4) The glasses presented in this patent can not only reduce the eyestrain during engaging in sustained near work, but also prevent myopia. For reason given above, these glasses can be extensively used regardless of old or young, and emmetropia, hyperopia or presbyopia.